Nice work!

I’ve done the same to two Rubberset 400s and a Moar Boar with a bad knot. I used approximately the same process as you for the Rubbersets, and also marred the ferrule, but upgraded my setup for the Moar Boar and I have worked out two recommendations for anyone trying to do this (or you if you do this again)

1. Use a vise or something to position the nozzle of a vacuum cleaner next to the handle. It catches the dust, and lets you see mutch better what you’re doing (and clean-up is easier, too, but doing it outside is still smart). Seeing better makes it easier not to hit the ferrule and gives more confidence while milling.
2. If you wrap the handle (especially the ferrule) in thick layers of cardboard and masking tape, you can hold the handle in a vise (still be careful not to crush it), and you won’t mar the ferrule if you accidentally touch it with the mill bit.

Pictures of my process are here

Next up for me is going to be extracting the shitknot of my Mühle Purist.